The use of positioning devices, particularly for systems measuring the radiation diagram of a transmitting antenna, is known. This positioning device can orient, for example, the transmitting antenna, in all directions relative to a fixed receiving antenna, with well-known features of a receiver device. This receiver device, which is connected to a data processing station, can pick up the electromagnetic field generated by the transmitting antenna some distance away, in every position or orientation of the antenna. In this way, the processing station can determine the radiation diagram of the transmitting antenna so as to determine the features of the antenna.
The transmitting antenna to be measured can be placed in a spherical element, so that the spherical element can be held by gravity, for example, on three support points of a positioning device. This positioning device of the prior art includes three drive wheels. The contact of each wheel on the external surface of the spherical element constitutes one of the three support points. These drive wheels can be small tyres, which are each set in motion by drive wheels, such as compressed air turbines or motors. The driving of the wheels causes the spherical element, in contact with the wheels, to rotate randomly in three directions.
With the use of three drive wheels, the positioning device of the prior art has some drawbacks. Indeed, when the three drive wheels rotate, the movement, which they communicate to the spherical element, involves a movement tangential to the support points, i.e. to the three contact points. This movement is the result of three wheel friction forces. In other words, the wheels skid most of the time on the external surface of the moving spherical element. The effect of this is to animate the spherical element in a chaotic manner.
The various frictions act like rubbers on the external surface of the spherical element, dirtying the external surface of the spherical element. Since, in some cases, the tangential movement of the sphere is exactly orthogonal to the orientation of one of the wheels, this therefore means that the spherical element jumps around its origin. This momentary deviation of the centre of the sphere, leads to an error in the measured position or orientation of the transmitting antenna, which constitutes a drawback. Moreover, during the various jumps, the spherical element, which may be off-balance, has a marked tendency to fall back along its mass centre. This results in a lack of homogeneity in the positions travelled.
U.S. Pat. No. 3,441,936 describes a device for positioning a spherical element in which a transmitting antenna can be placed. This spherical element is held, by gravity, on three drive wheels of the positioning device that are capable of orienting the transmitting antenna in any direction. When one drive wheel is being driven, in order to move the spherical element, the spherical element skids or jumps onto the other two drive wheels. This means that proper positioning of the transmitting antenna cannot be guaranteed, which is a drawback.